Apractice by southern state democratic parties in the nineteenth and early twentieth centuries intended to disenfranchise blacks by preventing them from voting in the crucial democratic party primary elections was known as the:

the white primary

Explanation:

Based on the information provided within the question it can be said that the crucial Democratic Party primary elections was known as the white primary. Like mentioned in the question this was one of the main methods used by White Democrats in order to disenfranchise most blacks from voting but also other minority voters as well. This usually took place between 1890 to 1908 and in all the states of the former Confederacy.